A full telephone number is usually made up of a country code (required for international calls only), area code (required for calls between telephone areas), prefix, and subscriber number.
As telephone technology advanced, the precise significance of the prefix became blurred in many places; for instance, 485 in London, UK, was once the GULliver exchange, but now 44-20-7485-xxxx is just considered one of many number blocks served by the CLKEN Kentish Town exchange.
As progress was made, exchanges were connected together, initially connected by the operator by name, and later dialed by users with prefixes such as WHI (for the WHItehall exchange, hence the famous Whitehall 1212 number for Scotland Yard), with letters corresponding to numbers on the dial (WHI was equivalent to # 944), later replaced by the numerical prefixes which remain in use.
When the song 867-5309/Jenny was released, hundreds of calls were made to various people and organizations in the U.S. with that number.
In the film Bruce Almighty a phone number belonging to a real user was shown, leading to a lawsuit.